1. Field of the Invention
The present invention relates generally to the field of immunology and vaccine development. More specifically, the present invention provides various immunogenic compositions comprising progastrin and their use in the prevention and treatment of cancer.
2. Description of the Related Art
Colorectal cancers are one of the most common forms of cancers in men and women in the US and one of the leading causes of death. The sequence of genetic events that are associated with the multi-step process of developing different types of familial colon cancers are well-known. Although advances in molecular genetics have led to better risk assessment and earlier diagnosis of colorectal cancer, it still remains a deadly disease for majority of the patients due to lack of effective adjuvant treatments. Such adjuvant or systemic treatments are likely to arise from a better understanding of factors that regulate proliferation of colonocytes and colon cancer cells. Just as genetic instability due to the inheritance of specific genetic defects plays a dominant role in initiation and progression of familial cancers, hyperproliferation is likely to play a permissive role in the initiation and progression of sporadic cancers. Hence, hyperproliferation is recognized as a risk factor that can initiate dysplastic growth, resulting in accumulation of genetic defects and progression to colon cancer. Gastrins represent a group of growth factors that can potentially play a prominent role in proliferation of normal and cancerous intestinal cells.
The gastrin gene is normally expressed and processed in the brain and in the antral stomach of mammalian species. The full length progastrin (PG) peptide (80 amino acids) undergoes enzymatic deletions both at the C and N terminal ends, to finally generate the fully processed C-terminally amidated gastrin peptides in the neuro endocrine cells (FIG. 1A). G17 and G34 amino acid gastrin peptides stimulate acid secretion and growth of the gastrointestinal (GI) tract. Although the colon cancer cells express the gastrin gene, they do not process the progastrin peptides (Singh et al., 2000a; Rengifo-Cam and Singh, 2004). Thus, patients with colorectal cancers (CRCs) are positive for significant levels of progastrin-like peptides in the circulation. Some studies have reported the presence of elevated levels of progastrin peptides but not gly-extended gastrin or gastrins in patients with colorectal cancers (Siddheshwar et al., 2001).
Additionally, a significant percentage of human colon cancer (HCC) cells have also been shown to require the expression of progastrin-like peptides for maintaining the in vitro and in vivo growth of the cells (Singh et al., 1996). Downregulation of the gastrin gene resulted in the attenuation of the growth of gastrin dependent human colon cancer cells in vitro and in vivo (Singh et al., 1996). A processing intermediate, gly-extended gastrin (GG) was reported to exert potent growth factor effects on several target cells including normal and cancerous intestinal epithelial cells (IEC) (Seva et al., 1994; Singh et al., 1995). Additionally, U.S. Pat. No. 5,786,213 and U.S. Pat. No. 6,165,990 disclose gene therapy of colorectal cancers using the anti-sense technology. However, the delivery of the anti-sense plasmids has not advanced significantly and has remained a concern along with associated side effects.
The use of immunotherapy, on the other hand, has advanced significantly and is currently being used for treatment purposes of many cancers by targeting other cancer-related molecules such as EGF receptors, HER-2 Neu Oncogene, Anti CD52, anti VEGF, Anti CD22, Anti CD80 etc. (Rosenberg et al., 2004). A vaccine against the gastrin peptide (G17DT) was developed and has been used in clinical trials with ambiguous results (He and Marshall, 2006). The G17DT vaccine is a chemical conjugate of 9 N-terminal amino acids of G17 sequence conjugated to DT (Diptheria Toxin). Since it is difficult to have a uniform conjugate with similar composition and stoichiometric ratios of peptide to carrier, the immune response would vary from batch to batch of vaccine. Additionally, peptide vaccines produce limited immune response. Another problem with the DT vaccine is that it may produce immunosuppression against the DT-peptide sequences as most of the humans are immunized against Diptheria. Furthermore, the quality control and quality assurance of conjugate vaccine is difficult.
It is known that several receptor (R) sub-types mediate the biological functions of gastrin-like peptides and progastrin-like peptides. Of these, CCK2R and its splice variants mediate biological effects of primarily CCK and gastrin-like peptides. On the other hand, novel proteins such as Annexin II mediate growth factor effects of progastrin-like peptides (Singh et al., 2007). The G17 N-terminal based vaccine essentially targets G17 and thus inhibits binding to CCK2R; progastrin based vaccine, on the other hand, may target the actions of progastrin peptides and inhibit binding to receptors such as Annexin-II. This may explain the high efficacy of the current vaccine.
Thus, prior art is deficient in an immunogenic composition that can be useful as vaccine in the treatment of cancers that produce progastrin ectopically or are dependent on progastrin for their growth. The current invention fulfils this long standing need in the art.